N.Korean Nuclear Site 'Was No Longer Usable'

North Korea's destruction of its Punggye-ri nuclear test site came after such a severe underground collapse that the site was in any case no longer usable, a recent study suggests.

The study published in the journal Seismological Research Letters claims that an aftershock observed after North Korea's last nuclear test in September last year was due to the collapse of a void as long as 80 m created after the explosion at Punggye-ri.

The test itself caused an artificial 5.7 magnitude earthquake, and the aftershock registered 4.5 on the Richter scale, causing experts to suspect a major ground collapse.

Japan's Yomiuri Shimbun on Wednesday said North Korea unveiled parts of demolition of the Punggye-ri nuclear test site to the media in May, but the new study shows that the site "may have been already unusable for further nuclear tests."

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